1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride-based polymer, and more particularly to a process for producing a vinyl chloride-based polymer with a high porosity, favorable plasticizer absorption, extremely low levels of fish eyes, and a high bulk specific gravity.
2. Description of the Prior Art
Vinyl chloride-based polymers are very useful resins that display excellent physical properties, and both hard and soft polymers are used in a wide variety of applications. In those fields where soft polymers are used, a variety of plasticizers are also used, meaning one of the important characteristics required of the vinyl chloride-based polymer is superior plasticizer absorption. In order to improve plasticizer absorption, the particles of the vinyl chloride-based polymer must be made more porous.
A multitude of processes for producing vinyl chloride-based polymers with excellent plasticizer absorption have already been proposed. For example, a process has been proposed for producing a porous vinyl chloride-based polymer by conducting a suspension polymerization of vinyl chloride in the presence of a partially saponified polyvinyl alcohol with a saponification degree of 37 to 70 mol % and a polymerization degree of 160 to 500 (see patent reference 1).
Furthermore, a process has also been proposed for producing a vinyl chloride-based polymer with superior plasticizer absorption by using (A) a partially saponified polyvinyl alcohol with an average polymerization degree of 1500 to 2700, and a saponification degree of 75 to 85 mol %, and (B) hydroxypropylmethylcellulose with a degree of methoxy group substitution of 26 to 30% by mass, a degree of hydroxypropoxy group substitution of 4 to 15% by mass, and a viscosity at 20° C. for a 2% by mass aqueous solution thereof of 5 to 4000 mPa·s as dispersion stabilizers, and then adding (C) a partially saponified polyvinyl alcohol with an average polymerization degree of 150 to 600, and a saponification degree of 20 to 55 mol % to the polymerization reaction system when the polymerization conversion is within a range from 5 to 50% (see patent reference 2).
In recent years, the potential applications for vinyl chloride-based polymers have become even more diverse, and the variety of plasticizers used has also expanded. Particularly in those cases where polymer-based plasticizers such as polyesters are used, vinyl chloride-based polymers with even better levels of plasticizer absorption than have conventionally been required are now becoming necessary.
Typically, as the level of plasticizer absorption is improved, a corresponding reduction in the bulk specific gravity tends to be observed. However, marked reductions in the bulk specific gravity are undesirable.
According to these processes described in the conventional technology, vinyl chloride-based polymers with a certain level of porosity and favorable plasticizer absorption can be obtained. However, if a large quantity of a polyvinyl alcohol with a low saponification degree is used in the early stages of the polymerization, with the intention of producing a vinyl chloride-based polymer with even better plasticizer absorption, then the protective colloid provided on the surface of the monomer droplets immediately following polymerization initiation is rapidly degraded, meaning the product polymer tends to form as coarse particles or lumps. Furthermore, foaming becomes more vigorous during recovery of the unreacted monomer, thus lengthening the time required for the unreacted monomer recovery step, and causing a marked deterioration in the productivity.
On the other hand, if the process proposed in the aforementioned patent reference 2 is followed, and a large quantity of a polyvinyl alcohol with a low saponification degree is added partway through the polymerization, then although the problem of the product polymer forming as coarse particles or lumps does not arise, the quantity of residual low saponification degree polyvinyl alcohol that remains within the liquid phase of the polymer slurry following completion of the polymerization increases, meaning foaming becomes more vigorous during recovery of the unreacted monomer, causing a marked deterioration in the productivity. Furthermore, although a vinyl chloride-based polymer produced by this process displays improved plasticizer absorption, the reduction in the bulk specific gravity is undesirably large.
[Patent Reference 1] GB Patent No. 1,524,492
[Patent Reference 2] U.S. Pat. No. 5,204,421